Use UART0 RX/TX as SCU UART debug port on iMX8QXP boards

On latest iMX8QXP MEK board, the hardware connected the SCU_GPIO0_00 and SCU_GPIO0_01 pins for SCU debug UART, and customer can enable "#define ALT_DEBUG_SCU_UART" from "imx-scfw-porting-kit-1.1/src/scfw_export_mx8qx_b0/platform/board/mx8qx_mek/board.c" to open the SCFW debug UART for early board bring up.

And if customer enabled "#define ALT_DEBUG_UART" from board.c, then SCFW will use ADC_IN2 and ADC_IN3 pins for debug UART.

In this document, it is another choice, SCFW can also use UART0_RX and UART0_TX pins as SCU debug UART for early board bring up. It is based on released "imx-scfw-porting-kit-1.1.tar.gz".

That means on early MEK boards and customer boards which haven't reserved debug UART for SCU, they can also check the SCFW boot log from UART0 port.

"scfw-porting-kit-1.1-sc_uart-on-uart0.patch" is the reference patch for such modification. Enable "#define ALT_DEBUG_SCU_UART_ON_UART0" to make it work.

Note: since UART0 pins had been used in SCFW, they can't be used in UBoot and linux kernel at the same time, so when debuging UBoot and Linux kernel, you need disable "ALT_DEBUG_SCU_UART_ON_UART0" in SCFW, or you can use other UART port and pins.